WARNING: there are no spoilers here! Seriously. This game has zero plot. The backstory is... up to you to find out.
Ingress is an augmented reality real-time massively multiplayer online capture-the-flag game. If you got through that mouthful, awesome. It's been developed by Niantic, an offshoot of Google, and hence the game is (currently) only available on Android. The premise is that aliens named Shapers have opened up portals here on Earth, and are slowly leaking in this stuff called "Exotic Matter" (XM). Beyond this bit, the Shapers do nothing. XM is virtually unknown to humans, except that it is both energy and matter (in the same way that water is solid, liquid and gas - from the game's own words). Not quite knowing what the Shapers' intentions are, humankind has split into two warring factions who each try to claim control over the portals. The Enlightened, represented in green, believe that this XM (and hence the Shapers) are ultimately out to benefit mankind, whereas the Resistance, represented in blue, believe the polar opposite. Players pick a faction and go from there.
The portals are worldwide, and are located at works of art, buildings of interest, etc. (but mostly in urban areas). Linking several portals together by "hacking" and obtaining the portals' Keys creates Control Fields. This can turn large portions of the map (remember, this is real-world) blue or green, depending on the faction that controls the portals. Of course, since this is capture the flag style gameplay, weapons are available to use on the opposing faction's portals to remove their control and claim it for your own.
It's a thinly-disguised attempt by Google to find out more about places of interest worldwide, by letting us peons do all the work for them. And it works! Not only that, but similarly to geocaching, it allows people to meet up with other like-minded people and to find places they had never visited or even knew about. For that alone, it gets a nice big fat tick from me. The teamwork aspect is also very well done: players must deploy "resonators" around a portal to claim it - one is sufficient for claiming but eight are necessary for linking - and the game mechanics restrict how many resonators a single player can place at a single portal1. The fact that this game has no plot is very forgivable given the nature of the game; the fact that there is a backstory being slowly released by Niantic keeps people compelled to play and follow the community.
There is one drawback. The level cap at the moment is 8. Even without any form of grinding, in high-density areas it is possible to reach the level cap quite quickly. From there it is relatively easy to lose interest. Thankfully, I am still at Lv3 (thanks largely to lack of mobile device with a data connection) and I am still sufficiently interested in the game. A lot of Lv8 players have circumvented this drawback by inventing their own goals (such as field art) and becoming actively involved in the community, but it still remains an issue.
The game is currently in closed beta. An invite is required, and a Google account is necessary. Despite this status, it is still played by millions worldwide. Viral marketing campaigns are largely responsible.
This wouldn't be a video game review without me going and giving it numbers, now would it?
It's been done well. The only real discrepancy could be your device and how well it performs... or doesn't.
Sound: 7/10 Minimalist, but with a sense of urgency. Can get a bit annoying, and it is recommended that you wear headphones or earbuds.
Playability: 8/10 It can be a bit intimidating to pick up, but the community is very helpful.
Lastability: 6/10 Progress through the game is largely self-motivated once you hit the level cap. It helps to have a good real-world rival (or eight).
Plot: The fun isn't in the plot. The fun is in the journey.
Total: 29.5/40 = 7.375/10 Not too bad. Needs a bit of technology to fully enjoy, but it's becoming more available.
1As an example: to create a Level 8 portal (the highest level thus far), it is necessary to have eight Lv8 resonators around it; since players can place only one Lv8 resonator at a single portal, eight players from the same faction are necessary for this condition to be met.