In allene, carbon atoms 1 and 3 are sp2-hybridised as each one of them is joined as a double bond. And, carbon atom 2 is sp-hybridised as it has two double bonds at each of its side. Therefore, the two -bonds are perpendicular to each other, in allene as shown below.
Ha and Hb lie in the plane of paper while Hc and Hd lie in a plane perpendicular to the plane of the paper. Hence, the allene molecule as a whole is non-planar.
