In Day of the Tentacle, you play as Hoagie in the past, as Bernard in the present, and as Laverne in the future. You never control two characters who are on the same screen.

Or, at least, that was the intended behavior. As explained in part 3, you can have two characters on the same screen. When they're together, they can exchange items without using the Chron-O-John. And, when that happens, the game things you've just picked up the item, only you aren't in the room where you're supposed to pick it up. Then, strange things happen.

This section will be about the way this glitch behaves when running the game on ScummVM. Check part 12: differences between ScummVM and other platforms for an explanation about how this glitch behaves on other platforms.

Quantum-swapping items

Let's say Bernard and Hoagie are in the same room, and Bernard's got the "help wanted" sign. Switch to Bernard, move to the spot where Hoagie is, click on the "help wanted sign", and press "1" to switch to Hoagie. Now you'll be controlling Hoagie, but the cursor will still be the "help wanted" sign! You can now either click on an item in Hoagie's inventory, or on the faces of the characters on the right of the inventory. When you do so, Hoagie will pick up the "help wanted" sign.

Well, that was certainly not exciting. That's what happens most of the time when you try to do this: either the characters refuse to pick up the item, or they pick it up with no special consequences. In any case, the quantum-swapping useless items video shows how Hoagie picks up the item.

Fortunately, there are some exceptions. The main ones are the patent application (also known as the battery plans), the crowbar, the right-handed hammer, and the contract. We'll talk about the patent application, the right-handed hammer and the crowbar in part 7. For now, let's focus on the contract. In particular, let's focus on what happens when you quantum-swap the contract at Red's basement in the past.

For a complete list of things you can do by quantum-swapping items, check the Day of the Tentacle notes on The Cutting Room Floor. Look for the word "quantum".

The battery ingredients skip

Quantum-swapping the contract when Dr. Fred is in the basement or when he's tied up in the attic won't accomplish anything. This trick will only work when Dr. Fred is in his office, opening and closing the safe while sleepwalking. Since you need the contract for this trick, you have to pick up the contract before giving Dr. Fred the decaf coffee. For an example of how to do so, check the section about the canary conversation in part 2.

This trick will skip two of the three battery ingredients. Since the oil is by far the easiest one to get, before doing this trick, pick up the oil.

Let's say Hoagie and Laverne are in the past, and Laverne's got the contract. Get both of them to Red's basement and give the patent application to Red. Now, move Hoagie and Laverne to the same spot. Switch to Laverne, click on the contract, press "1" to switch to Hoagie, and click on an item in Hoagie's inventory (you can also click on Laverne's/Bernard's faces on the right of the inventory).

Once you do so, wait, and... the game will crash. Oops. Well, nobody said it was going to be easy.

What's happening here is that, when quantum-swapping the contract, the game will think you're giving one of the battery ingredients to Red. After that, the game will try to remove that ingredient from your inventory. But you aren't actually giving Red an ingredient, so, when trying to remove it from your inventory, the game will crash.

Luckily, there's a way to avoid the crash. After quantum-swapping the contract, right when Red turns in order to get the item you're supposedly giving him, switch to a character who's not on the screen (in this case, Bernard). You can also do it during the frame right before Red turns to get the item. The timing is tricky, so you'll have to practice.

Now, switch back to Laverne, and once more, click on the contract, press "1" to switch to Hoagie, click on an item in Hoagie's inventory, and switch to Bernard.

If you've successfully done the trick both times, when you give the oil to Red, he'll make the battery, as you can see in the battery ingredients skip video.

Take into account that Red expects you to give him three items. Not at least three items, but exactly three items. So, if you quantum-swap the contract three or more times, and then you give him the oil, he won't make the battery. And you'll softlock the game.

Under the hood: pick-up scripts and room script interruptions

When you click on an item in your inventory, the cursor becomes that item. If then you switch to a different character, and that character is in the same room, the cursor will still be the item. As an example, let's assume you're controlling Laverne, you click on an item in Laverne's inventory, and then you switch to Hoagie.

Hoagie doesn't have the item, so if you click on another item in his inventory, Hoagie will try to first pick up the item, and then use it with the one in his inventory. Similarly, if you click on the portraits on the right of the inventory, you're asking Hoagie to give an item he doesn't already have, so he'll try to pick it up first.

And where is the item Hoagie is trying to pick up? Well, since the item is in Laverne's inventory, it's where Laverne is. That's why the two characters need to be on the same spot (or really close). Otherwise, Hoagie will say "I can't reach it", and won't pick it up.

After you pick up an item, most of the time nothing special happens. You get the item, the item disappears from its original location, and that's it. However, some items start room scripts when you pick them up (the patent application actually starts a global script, as we'll see in part 7).

For example, when you try to pick up the crowbar, the game starts room script 207, which makes the man in ski mask say "Hey, do you mind? I'm using this". Or, when you try to pick up the contract with Dr. Fred in his office, the game starts room script 205, which makes Dr. Fred close the safe.

But, what happens when you pick up these items in a different room? Picking up the crowbar will start room script 207 in the room you're currently in, while picking up the contract will start script 205 in the room you're currently in.

In part 1, we said that, if you picked up the contract while you're in the kennel in the future, Laverne would give the dinner certificate to the guard. You can do this by quantum-swapping the contract. Of course, for that, you'll have to get someone else to the future. We'll see how in part 5 and part 7.

The ingredients skip is a bit peculiar. When you give an ingredient to Red, the game first increases the counter of the number of ingredients you've given him. Then there's the animation of Red getting the item, and then the game removes the item from your inventory. So, if you switch to a different character while Hoagie is giving Red an ingredient, you'll keep the item and it'll still count as if you've given an item to Red.

In theory, you could use this to get the battery by giving Red the oil three times in a row. However, there's no known way to switch to a different character while Hoagie is legitimately giving Red the oil.

That's where quantum-swapping the contract comes in handy. When you do so, the game will start room script 205 (if Dr. Fred is in his office). In the past basement, room script 205 is Hoagie giving an ingredient to Red. And, when you quantum-swap the contract, you keep control of your character during Red's animation.

So you can switch to a character who's in a different room before the game tries to remove the ingredient from your inventory. Since room scripts stop running when you leave the room, room script 205 will stop running, the game won't try to remove the ingredient from your inventory, and the game won't crash.