Stream stream = null;
try
{
    plugin.Initialize(); //plugin is IPlugin instance created with Activator.CreateInstance

    stream = File.Open("serialized.txt", FileMode.Create);
    BinaryFormatter formatter = new BinaryFormatter();

    formatter.Serialize(stream, plugin); //Successful
    stream.Close();

    stream = File.Open("serialized.txt", FileMode.Open);
    BinaryFormatter formatter2 = new BinaryFormatter();

    object obj = formatter2.Deserialize(stream); //Fails

    this.Text = obj.GetType().ToString();
}
catch (Exception ex)
{
    this.Text = ex.Message;
}
finally
{
    stream.Close();
}