In the middle of the night Trojan guards on the lookout for suspicious enemy activity sight bright fires in the Greek camp. They promptly inform Hector, who almost issues a general call to arms before Aeneas makes him see how ill-advised this would be. Their best bet, Aeneas argues, would be to send someone to spy on the Greek camp and see what the enemy is up to. Dolon volunteers to spy on the Greeks in exchange for Achilles's horses when the war is won. Hector accepts the deal and sends him out. Dolon leaves wearing the skin of a wolf, and plans on deceiving the Greeks by walking on all fours. Rhesus, the neighboring king of Thrace, arrives to assist the Trojans soon after Dolon sets out. Hector berates him for coming so many years late, but decides better late than never. Rhesus says he intended on coming in the beginning, but was sidetracked defending his own land from an attack by Scythians.
Meanwhile, on their way into the Trojan encampment, Odysseus and Diomedes run into Dolon and kill him. When they reach the encampment with the intention of killing Hector, Athena guides them to Rhesus' sleeping quarters instead, pointing out that they are not destined to kill Hector. Diomedes slays Rhesus and others while Odysseus takes his prized horses before making their escape. Rumors spread from Rhesus' men that it was an inside job, and that Hector was responsible. Hector arrives to cast blame on the sentinels for, due to the sly tactics, the guilty party could only be Odysseus. The mother of Rhesus, one of the nine muses, then arrives and lays blame on all those responsible: Odysseus, Diomedes, and Athena. She also announces the imminent resurrection of Rhesus, who will become immortal but will be sent to live in an underground cave.
This short play is most notable in comparison with the Iliad. The part with Dolon is pushed to the background, and much more is revealed about Rhesus and the reactions of the Trojans to his murder.

Question: Why was the king of Thrace, Rhesus, delayed in coming to Hector's aid?
Answer: He was defending his kingdom from Scythians.
Alternative answer: He was defending his lands from the Scythians