Females of many species mate with multiple males (polyandry), resulting in male-male competition extending to post copulation (sperm competition). Males adapt to such postcopulatory sexual selection by altering features of their ejaculate that increase its competitiveness, and/or by decreasing the risk of sperm competition through female manipulation or interference with rival male behaviour. At ejaculation, males of many species deposit copulatory plugs, which are commonly interpreted as a male adaptation to postcopulatory competition, and are thought to reduce or delay female remating. Here, we used a vertebrate model species, the house mouse, to study the consequences of copulatory plugs for postcopulatory competition. We experimentally manipulated plugs after a female's first mating and investigated consequences for rival male behaviour and paternity outcome. We found that even intact copulatory plugs were ineffective at preventing female remating, but that plugs influenced rival male copulatory behaviour. Rivals facing intact copulatory plugs performed more but shorter copulations and ejaculated later than when the plug had been fully or partially removed. This suggests that the copulatory plug represents a considerable physical barrier to rival males. The paternity share of first males increased with a longer delay between the first and second males’ ejaculations, indicative of fitness consequences of copulatory plugs. However, when males provided little copulatory stimulation the incidence of pregnancy failure increased, representing a potential benefit of intense and repeated copulation besides plug removal. We discuss potential mechanisms of how plugs influence sperm competition outcome and consequences for male copulatory behaviour.