Dennis is a clueless and slightly overweight guy. Five years ago, he left Libby, his pregnant fiancée, on their wedding day. Realizing he made a mistake, Dennis tries to persuade her that he loves her and wants her to accept him back into his life, but he meets with failure at every turn. When he discovers that Libby has found Whit, the new man in her life, the frustration Dennis feels grows. Denise vows, that for once in his life, he will finish something and decides to train for the Nike River-run in London. With his friends Gordon and Mr. Goshdashtidar by his side, Dennis begins training for the marathon he must finish.