I've never really gotten into playing custom levels for Portal 2 —I just don't find the game that much fun without the inclusion of Wheatley, Cave Johnson, and GLaDOS. That's why it's great when Portal 2 mods add something new to make up for what's missing. In this case, the added element is a hand-held time device that lets Chell make a time-shifted duplicate of herself, and team up with it to solve puzzles. Thinking with portals is no longer enough, now you're Thinking With Time Machine .
In Thinking With Time Machine, the time machine is a tablet you hold in your left hand and can view by looking down at it. Pressing R starts a recording: a recording of all your actions. Walking, jumping, crouching, standing still, even picking up or dropping objects or activating switches, the time machine will record it all. Pressing Q stops the recording. Pressing F starts a replay, wherein a duplicate of Chell—the Chell from the past—materializes and repeats the actions you—she—just recorded. It's a bit reminiscent of a game like P.B. Winterbottom , only you only get one clone at a time, and it takes place in Aperture Science.
It's very cool watching the past version of Chell appear and run through her recording. It's not like seeing yourself is particularly weird in Portal, what with all the portals giving you glimpses of yourself, but there's something neat about just standing there and watching your past-self go to work and then dematerialize when she's done. You can play the recording as many times as you want (or need), and each time you record something new, the previous recording is erased.
The time pad is wonderfully realized as well. At a glance, you can tell if you're recording or not (its indicator glows orange when you are), and during playback, not only does it count down how much time your recording has left, but you can actually see what past-Chell is seeing on your screen, which is helpful later when you need to play your recording while you're in a different room from your clone. (My only question is, why isn't past-Chell holding the time pad? I'm sure someone smart could come up an actual sci-fi answer.)
While you're enjoying the sight of your past self running around, and checking out the sleek beauty of your new time device, you'll probably notice something else new: you can look down and see your legs. I've always felt Valve has been behind the curve on moving past the "floating gun" protagonist, but this mod's added legs aren't just for show. It actually helps to see your legs, since you will, from time to time, be standing on your own shoulders.
Once you've gotten the hang of pushing a few extra buttons, the earliest puzzles aren't tough to figure out. If standing on a button opens a door, you record yourself standing on the button, then run to the door, and replay your past self standing on the button, allowing you to escape. If you need a cube placed on a switch while you're somewhere you can't do it yourself, like on the other side of a laser field, just record yourself doing it, and replay it when you're in position. Eventually, working with your past self will seem almost as natural as working with a co-op partner, including, oddly, the occasional feeling of impatience with your partner.
Things quickly get complicated. In Portal, solving a chamber usually involves a few steps: figure out what conditions need to be met to escape the chamber, come up with a plan to meet those conditions, and then perform the actual tasks. This mod adds another layer: performing the steps that will allow you to perform the steps, recording them, then playing them back while performing another set of steps. Some of these chambers are dastardly, but the novelty of summoning a past version of yourself helps to keep the frustration level tolerable, mostly.
This mod isn't just a collection of levels with a clever new gimmick, either. Right from the start, it makes an effort to tie itself into the existing Portal 2 story rather than just fading in on you holding the time-pad. The tutorials are handled nicely as well: in the first handful of levels you'll learn the ins-and-outs of time machine puzzles on video screens, which demonstrate how the whole thing works in wonderfully done animations. There's some on-screen help as well, as icons will appear giving you countdowns, showing you where you need to place a block so your recording can pick it up, and so on.
While it's fun watching your past self run through the motions you just ran through, it can get a little weird, too, like in one level where you leave your past self to perform some tasks while you wait in the chamber below. You can't see your doppleganger, but you can tell she's doing her job, and it's sort of eerie to think about her, you, running around up there unattended. You'll also spend some time passing objects between former you and current you, which is a bit trippy, especially since the first time you passed the object to yourself, you weren't yet there to receive it.
Another challenge, in one chamber, is performing a tricky, time-dependent task perfectly, not just once, but twice: once for your past-self to repeat, and once for your current self to complete while your past-self is replaying it. Usually, you only need to do a perfect run once in Portal 2, but this makes it twice as challenging.
If I have one complaint, it's that this Portal 2 mod features very little in the way of, y'know, portals. As it stands, I think there's only a couple puzzles that really involve the portal gun, and I would have loved to see more of them. Still, this is a wonderfully creative and well-executed mod. It's free if you own Portal 2, and you should absolutely try it.