The good features An Awfully Big Adventure has are the screenplay and the cast lineup. The plot doesn't have a lack of diversity or intrigue, per se, but it is a very eventful film. Any fan of Alan Rickman who has not seen this movie is surely missing out.
We start off the production with Stella (Georgina Cates), a 16-year-old girl, who lives in Liverpool with her aunt and uncle. She doesn't know much about her parents, however, we do see her frequently making calls in phone booths to talk with her mother. We do not get to see Stella's mother on film. Stella wants to be an actress, taking speech lessons which her uncle signs her up for. When we see Meredith Potter (Hugh Grant) on screen, we see him as arrogant, perhaps even callous. Stella does not see him this way, admiring everything he does, as she is only 16 and is very impressionable and vulnerable. Stella becomes caught up in things behind the curtains but is quickly made a victim of advances by the older men in the theatre comapny. One of these men is P.L. O'Hara (Alan Rickman), a well-known actor who returns for the role of Captain Hook in a Christmas production of Peter Pan. P.L. is seen as charming, almost legendary, by the other members of the cast, when he is really just as troubled as the rest of them. O'Hara believes that his lost love bore him a son, but he sets everything aside to be with Stella; he feels a deep emotional connection to her. Stella is still emotionally connected to Meredith, but uses P.L.'s advances to gain experience.
The final turn of the knife in this movie is the underlying plot twist that nobody really could have foretold. O'Hara goes to visit Stella's aunt and uncle to learn more about Stella's past when he sees a photo frame of Stella and her mother. It then hits him: the reason he felt so connected to Stella was that she was his daughter. His 'lost love' did not bear him a son, and it was his daughter he had been with all along. P.L. tells nobody about this and rides out to the coast on his motorcycle. He, unfortunately, slips on a gangplank, causing him to fall and hit his head. O'Hara falls underwater, causing him to drown and die.
TL;DR: Alan Rickman's character falls in love with his daughter (blissfully unaware it's her), which eventually leads him to suicide.