There are several correlations or similarities between the two lives, but I think plagiarism is too strong a word. I think the Lord manifested Himself in both persons and choose to make the lives similar. One could also argue the scientific axiom "Correlation is not causation" - simply because two events exhibit similarities does not mean that one is the cause of the other.
Swami Vivekananda says (Complete Works, V1, Lectures and Discourses, Krishna also here - http://cwsv.belurmath.org/volume_1/vol_1_frame.htm):
A few words about the life of Krishna. There is a great deal of similarity between the lives of Jesus and Krishna. A discussion is going on as to which borrowed of the other. There was the tyrannical king in both places. Both were born in a manger. The parents were bound in both cases. Both were saved by angels. In both cases all the boys born in that year were killed. The childhood is the same. ... Again, in the end, both were killed. Krishna was killed by accident; he took the man who killed him to heaven. Christ was killed, and blessed the robber and took him to heaven.
There are a great many similarities in of the New Testament and the Gita. The human thought goes the same way. ... I will find you the answer in the words of Krishna himself: "Whenever virtue subsides and irreligion prevails, I come down. Again and again I come. Therefore, whenever thou seest a great soul struggling to uplift mankind, know that I am come, and worship. ..."(Gita IV. 8; X. 41.)
At the same time, if he comes as Jesus or as Buddha, why is there so much schism? The preachings must be followed! A Hindu devotee would say: It is God himself who became Christ and Krishna and Buddha and all these [great teachers]. A Hindu philosopher would say: These are the great souls; they are already free. And though free, they refuse to accept their liberation while the whole world is suffering. They come again and again, take a human embodiment and help mankind. They know from their childhood what they are and what they come for. ... They do not come through bondage like we do. ... They come out of their own free will, and cannot help having tremendous spiritual power. We cannot resist it. The vast mass of mankind is dragged into the whirlpool of spirituality, and the vibration goes on and on because one of these [great souls] gives a push. So it continues until all mankind is liberated and the play of this planet is finished.
Some people have seen similarities between some of the practices also. It is difficult to say if one is derived from the other as any evidence has been lost in time and there is a certain commonality to the human experience. But as some examples, Christians have their Eucharist which in their ceremony food is offered to God and then given to the individuals to eat. In pujas, Hindus offer food to God and then it is given to them to eat as prasadam. Additionally, there are all the other elaborate elements of ritualistic worship that are similar (incense, lights, etc.).