You Do Need power plug adapter while traveling from São Tomé and Príncipe to Saint Helena.

Here is Why You Need An Adapter

The outlet and plug type used in Saint Helena is G and in São Tomé and Príncipe are C and F. This is how they look like.

São Tomé and Príncipe

In São Tomé and Príncipe, outlets of type C and F are in use.

Type C

Type F

Saint Helena

In Saint Helena, outlets of type G are in use.

Type G