Project Excalibur was the codename used by the British government to refer to a secret nuclear missile launch facility hidden under Blairquhan Castle on an island off the coast of Scotland.
The facility in Scotland held a single nuclear ballistic missile that was capable of destroying a major city. It was intended as a weapon of last resort but fortunately never deployed. Two government agents, Alfred Pennyworth and Frederick, were each entrusted with one half of the launch code, making them the only two men in the world able to launch the missile.
Many years after Alfred's retirement from the service, the terrorist known as Red Claw discovered the facility and kidnapped both men, intending to get the code and use the missile to destroy London.
Unknown to her, Alfred's current employers, Bruce Wayne and Dick Grayson were secretly Batman and Robin. Bruce, aware of Alfred's intelligence background, deduced from the coded note that Alfred left that the problems he referred to could have something to do with his past and thus could have international implications. Following Alfred's trail, the pair managed to learn the truth and stormed the castle, fighting their way through many of Red Claw's henchmen.
Meanwhile, Red Claw herself drugged her prisoners with truth serum and questioned them about the code. Frederick gave up his half relatively quickly but Alfred managed to resist much longer by reciting quotations from British verse. This was both to resist the drug and to cover the fact the opening lines from the nursery rhyme The Lion and the Unicorn were, in fact, the code. Thus when his resistance inevitably broke down, it wasn't immediately obvious.
Red Claw realized Alfred's deception seconds before Batman and Robin broke into the control room and successfully launched the missile. Leaving Robin to free the pair and deal with the remaining thugs, Batman flew off in the Batwing to catch the missile before it reached its target. He faced resistance from Red Claw who had hidden herself in the Batwing's rear seat but, after a brief struggle, ejected her into the ocean and destroyed the missile with seconds to spare.