Increased commodity use of mobile devices has the potential to enable mission-critical monitoring applications. However, these mobile-enabled monitoring applications have to often work in environments where a delay-tolerant network (DTN) is the only feasible communication paradigm. Detection of complex (composite) events is fundamental to monitoring applications. However, the existing plan-based CED techniques are mostly centralized, and hence are inherently unscalable for DTNs. In this paper, we create Comet - a decentralized plan-based, efficient and scalable CED for DTNs. Comet shares the task of detecting complex events (CEs) among multiple nodes, with each node detecting a part of the CE by aggregating two or more primitive events or sub-CEs. Comet uses a unique h-function to construct cost and delay efficient CED trees. As finding an optimal CED plan requires exponential-time, Comet finds near-optimal detection plans for individual CEs through a novel multi-level push-pull conversion algorithm. Performance results show that Comet reduces cost by up to 89% compared to pushing all primitive events and over 60% compared to a two-level exhaustive search algorithm.