You Do Need power plug adapter while traveling from Tajikistan to Saint Helena.

Here is Why You Need An Adapter

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

Tajikistan

In Tajikistan, 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