In the fifth aliyah Bilam sends word with Balak's messengers that G-D doesn't give him permission to go with them. Actually, Rashi says that Bilam is hinting that he would go with messengers who are more honorable than those sent the first time. Balak then sends more honorable messengers to Bilam and promises him fancy rewards. Bilam discusses the matter again with G-D, and now G-D says that he may go, but only to speak such words that G-D tells him to.
Bilam saddles his donkey and goes with the Moavite dignitaries. However, G-D places an angel with a drawn sword in Bilam's way. Rashi says that animals have better sight than humans, so the donkey could see the angel while Bilam could not. The Torah describes the trouble Bilam has trying to get the donkey to go forward. G-D finally gives Bilam the ability to see the angel and Bilam sees him and becomes afraid and offers to go back. But the angel says he can go on, but only to say what the G-D wants him to say.