It's no spoiler telling you that the idea in
Chicken Run is to save chickens by leading them out of their incarceration, much like the movie. Playing as Rocky, you use chicken feed to lure hens past all kind of danger in 20 different levels. Levels are split between The Farmyard and The Pie Machine, growing steadily more challenging. Before each mission, Rocky gets a briefing from Mac on dangers that will show up in the next level. Also, in a very 'James Bond' way, Mac plays 'Q' in developing tools Rocky can use to overcome obstacles in levels and bypass Tweedy's fortifications.
In The Farmyard, most obstacles involve roaming dogs, spotlights and electrified fences or barbed wire. Some of Mac's devices include a common spoon that Rocky can use to dig under fences, boxes to put in front of sleeping dogs and instructions on disarming the searchlights. The hens will follow a trail of chicken feed, but once they get on track, they'll also go back the way they came. As Rocky goes through disarming traps and making progress through the level, he'll need to also go back and pick up feed to keep the hens moving. Some areas include extra bags of feed in case you run low, but recycling also solves the shortage problem. The Pie Factory has more high-tech devices, including cameras, laser fences and metal grates. Rocky can use a wrench on the gates, but cameras require deflection by mirrors that Mac provides and key cards disarm the laser gates. Also, a hilarious catapult item will send hens flying over an obstacle, if need be.
Rocky is able to jump over pipes or boxes, but the hens are grounded. This makes for good gameplay, since Rocky can't always lead the hens along the most direct route. Many times, you run ahead and prepare the route by disarming traps and spreading feed before leading the hens through. Two things make it something other than a pleasure cruise. First, there's a timer counting slowly downward. Second, all the surveillance and prowling dogs can alert a Tweedy guard to your escape party, if alerted. Getting through levels unseen and under time grows increasingly more difficult as you play, but that's what makes it fun.