Complex sentence: Race car driver Mike Reardon and mechanic Tommy Trehearn are the best of friends whose friendship is damaged, however, when Reardon's girlfriend (Chris) ends up falling for Trehearn.
Split into simpler sentences: Race car driver Mike Reardon and his mechanic Tommy Trehearn are the best of friends. Their friendship is damaged however, when Reardon's girlfriend ends up falling for Trehearn.